1. Technical Field
This invention relates to a valve device which is suitable for use in hydraulic control of a hydraulic apparatus such as an automatic transmission installed in a vehicle, and a failure detector for a hydraulic circuit comprising the valve device.
2. Description of the Related Art
Conventionally, as a valve device for switching flow passages of hydraulic fluid for gearshift control in an automatic transmission installed in a vehicle, a valve device comprising an actuator consisting of a solenoid is used. As shown, for example, in Japanese Patent Application Publication No. 2003-74729 (Patent Document 1), this type of valve device comprises a valve plug for opening and closing an oil passage and a solenoid for driving the valve plug intervened by a plunger, having a structure which drives the valve plug by turning on/off electromagnetic force of the solenoid to switch opening and closing of the oil passage. Specifically, in this type of valve device, the oil passage is closed when power is off because a spring-biased plunger pushes the valve plug to a valve seat surface. In contrast, the plunger retracts against the biased force of the spring by electromagnetic force of the solenoid when power is on. This causes the valve plug to separate from the seat surface by the force of hydraulic pressure of the hydraulic fluid and open the oil passage.